1. Field of the Invention
The present invention relates generally to optical devices, and more particularly to optical element alignment assemblies and methods of making the same.
2. Description of the Related Art
An optical component, such as a mirror, lens or fiber, in an optical instrument or device, such as an optical switch, should be accurately located/positioned with respect to another optical component in order for the optical instrument or device to function properly. Thus, optical devices may require their components to be placed with exacting tolerances to fulfill design objectives.
Conventional passive alignment assemblies for MicroElectroMechanical System (MEMS) devices are typically planar in nature and only align local elements, e.g., a fiber and ball lens collimator, where the two components are within a few millimeters of each other. Alignments over larger distances (e.g., greater than five millimeters), and three-dimensional optical systems typically use conventionally machined components. Such assemblies often fail to align optical components with high intrinsic precision.
Components generally need to be located in three dimensions, i.e., distributed in a volume of space, and have three rotations specified and/or controlled. Components located in a plane (two dimensions) with three or fewer rotations specified and/or controlled are a subset of the general case. Other design objectives may include: (1) locate components without induced strains, either from the process of mounting or through bulk temperature changes of constituent parts, and/or (2) support components as rigidly as possible.
In accordance with the present invention, alignment assemblies and methods of using and making the assemblies are provided. An important advantage of several embodiments of the invention is to completely orient one body with respect to another body to a high degree of precision by providing (1) precise mating features between bodies and connecting elements, and (2) precise distances between these features on all bodies and connecting elements.
In one embodiment, the alignment assemblies are passive, kinematic or nonkinematic, and micromachined. xe2x80x9cPassive alignmentxe2x80x9d means the various parts or devices to be assembled have mating features such that when these features are engaged with each other, the correct alignment (typically optical) is attained. In some instances, the engagement of these mating features permanently controls the alignment. In other instances, some type of fixture will hold the parts with their mating features engaged while some additional fixation, e.g., glue or bolt, is added to make the engagement permanent.
For comparison, in xe2x80x9cactivexe2x80x9d alignment, two parts or devices are maneuvered with respect to each other by some motion control mechanism, e.g., a motorized motion stage, shim set, etc., in one or more directions or degrees-of-freedom (DOF) until some metric, e.g., light through-put, optical beam quality, etc., is within a specified tolerance. At that point, the two parts are fixed rigidly with respect to each other by some means, e.g., glue, solder, bolt.
As defined and used herein, xe2x80x9ckinematic mountingxe2x80x9d relates to attaching two bodies, which may be called a base assembly or a payload assembly, together by forming a structural path and creating stiffness between the two bodies in six, and only six, independent degrees of freedom (xe2x80x9cDOFsxe2x80x9d) or directions. Each degree of freedom (DOF) kinematically controlled between two bodies is also a position defined, i.e., a specific value of that DOF, as a linear measurement, may be maintained. Six DOFs are desired because the location of any object in space is defined by three orthogonal coordinates, and the attitude of the object is defined by three orthogonal rotations.
A kinematic support has the advantage of being stiff, yet any strains or distortions in the base assembly are not communicated to the payload assembly. Thus, any sensitive optical alignments are not altered in the payload assembly if the base assembly undergoes deformation due to applied loads or bulk temperature changes.
In one embodiment, it is desirable to tailor a DOF based on the configuration of a xe2x80x9cpseudo-kinematicxe2x80x9d support. xe2x80x9cPseudo-kinematicxe2x80x9d means that although there may be many DOFs connecting at least two bodies, such as two micromachined passive alignment assemblies, in a practical attachment scheme, the DOFs can be tailored such that only six DOFs have a relatively high stiffness, and substantially all other DOFs have a relatively low stiffness.
Thus, true xe2x80x9ckinematicxe2x80x9d support means only 6 stiff DOFs connecting two parts, and no other stiffness paths exist. xe2x80x9cPseudo-kinematicxe2x80x9d means there are 6 DOFs with relatively high stiffness, and possibly many more with much lower stiffness (typically two to three orders of magnitude less). In some applications, it is desirable to have pseudo-kinematic DOFs with relatively low stiffness to be two to three orders of magnitude lower than DOFs with relatively high stiffness.
DOFs with different levels of stiffness may be accomplished using a flexure system to relieve stiffness in unwanted DOFs. Depending on the cross-sectional properties of elements in the flexure system, connecting elements between two bodies may attain the desired stiffness connectivities.
The alignment assemblies and methods of making the assemblies according to the invention may provide a number of advantages. For example, the micromachined passive alignment assemblies may be made with high intrinsic precision. Micromachining processes may form three-dimensional structures from a substrate wafer with high accuracy. In several embodiments, one micromachined passive alignment assembly may be oriented and spaced with respect to another assembly (e.g., with connecting elements) with lithographic precision, e.g., three-dimensional translational positioning to less than one micron and three-dimensional angular positioning to less than five arcseconds for an assembly with a 50-mm characteristic dimension.
The methods according to the invention may construct mating surfaces on micromachined passive alignment assemblies, such as a base assembly and a payload assembly, to control six independent DOFs between the assemblies and allow complete, high-precision specification of position and attitude. In some applications, it is desirable to have micromachined connecting elements with counterpart mating surfaces to mate with the mating surfaces on the base and payload assemblies.
The accuracy of micromachined passive alignment assemblies may be fully realized if there is a positive contact between a pair of mating features. Thus, some form of preload or force may be applied to maintain compressive contact between the pair of mating features. An external force may be applied to preload mating surfaces to contact each other prior to gluing. Glues that shrink on cure may be used to maintain the preload across mating surfaces after assembly.
In addition to or instead of an external force, any of the structural elements being assembled may have an internal flexure assembly that applies an internally-reacted force (preload). The internal flexure assembly may seat mating surfaces without a deadband. In one embodiment, the internal flexure assembly comprises a set of double parallel motion flexures, a preloader stage, and a hole on one side of the preloader stage for inserting a separate preloader pin. When the preloader pin is inserted into the hole of the internal flexure assembly, the preloader stage deflects and exerts a force on the pin, which exerts a preload against a mating surface. After the micromachined passive alignment assemblies are assembled, the mating surfaces may be glued or bonded if desired.
A connecting element may be configured to restrain the base assembly and the payload assembly with one or more desired DOFs. In some embodiments, a xe2x80x9cdegeneratexe2x80x9d support or connecting element may be used where less than six constrained DOFs between a base and payload are desired. The degenerate support may allow some trajectory (i.e., a combination of Cartesian DOFs) of a payload assembly relative to a base assembly to be unconstrained.
A xe2x80x9credundantxe2x80x9d support or connecting element may be used in applications where more than six DOFs are desired. The redundant support reinforces the base and payload assemblies and maintains their flatness.
As another example, a micromachined passive alignment assembly may have thermal compensation flexure assemblies for maintaining centration of optical elements in the presence of large bulk or local temperature differences. The optical elements may then be attached to at least three pads supported by these flexure assemblies to effect this stable positioning. In some applications, it is desirable to position a plurality of optical elements in a precise pattern in the presence of large bulk or local temperature differences. In some of these applications, it may be desirable to position a plurality of thermal compensation flexure assemblies concentric with respect to the center of an opening and equidistant with respect to each other.
One aspect of the invention relates to an assembly configured to support at least one optical element to a pre-determined position. The assembly comprises a first micromachined structure having at least a first mating part and a second micromachined structure having at least a second mating part. The second mating part is configured to contact the first mating part to constrain the second micromachined structure with respect to the first micromachined structure. The second micromachined structure is configured to support at least one optical element.
In one embodiment, the second mating part is configured to contact the first mating part to precisely position the second micromachined structure with respect to the first micromachined structure. In one embodiment, the optical element is then precisely positioned with respect to the first micromachined structure. In one embodiment, the first micromachined structure also supports one or more optical elements.
Another aspect of the invention relates to an assembly configured to support at least one optical element. The assembly comprises a first micromachined structure having at least a first attachment point and a second micromachined structure having at least a second attachment point. The second attachment point is configured to contact the first attachment point to restrain the second micromachined structure with respect to the first micromachined structure in at least one degree-of-freedom (DOF). The second micromachined structure is configured to support at least one optical element at a predetermined position.
In one embodiment, the second attachment point is configured to contact the first attachment point to restrain and align the second micromachined structure with respect to the first micromachined structure. In one embodiment, the optical element is then aligned to a pre-determined position with respect to the first micromachined structure. In one embodiment, the first micromachined structure also supports one or more optical elements.
Another aspect of the invention relates to a method of making an assembly configured to position an optical element to a pre-determined position. The method comprises using lithography to form a first pattern and a second pattern on a substrate for a first structure and a second structure. The first pattern outlines a first mating part of the first structure. The second pattern outlines a second mating part of the second structure. The method comprises etching the substrate to form the first and second structures according to the first and second patterns. The second mating part is configured to contact the first mating part to constrain the second structure with respect to the first structure. The second structure is configured to position at least one optical element.
One aspect of the invention relates to an assembly configured to support at least one optical element to a pre-determined position. The assembly comprises a micromachined base, a payload and a connecting structure. The base has a first mating part. The payload is configured to position the optical element. The payload has a second mating part. The connecting structure is configured to contact the first mating part of the base and the second mating part of the payload. The connecting structure constrains the payload in about five to about six degrees of freedom with respect to the base.
In one embodiment, the base also positions an optical element.
Another aspect of the invention relates to an assembly configured to position at least one optical element to a pre-determined position. The assembly comprises a base plate and at least one side plate configured to connect to the base plate. The base plate and the side plate are configured to support a plurality of payload plates. Each payload plate is configured to connect to the side plate and to the base plate. Each payload plate is configured to position at least one optical element.
Another aspect of the invention relates to a method of making an assembly configured to position at least one optical element to a predetermined position. The method comprises using lithography to form a first pattern, a second pattern and a third pattern on a substrate for a base, a payload and a connecting structure. The first pattern outlines a first mating part of the base. The second pattern outlines a second mating part of the payload. The third pattern outlines third and fourth mating parts of the connecting structure. The method further comprises etching the substrate to form the base, the payload and the connecting structure according to the first, second and third patterns. The connecting structure is configured to contact the first mating part of the base and the second mating part of the payload. The connecting structure constrains the payload in about five to about six degrees of freedom with respect to the base. The payload is configured to position an optical element.
One aspect of the invention relates to a micromachined flexure assembly formed in a first structure that is a part of an optical element alignment assembly. The flexure assembly comprises a set of parallel motion flexures and a preloader stage coupled to the set of parallel motion flexures. The set of parallel motion flexures allows the preloader stage to deflect away from a second structure of the optical element alignment assembly and apply a load against the second structure to constrain the second structure in at least one degree of freedom with respect to the first structure.
Another aspect of the invention relates to a micromachined thermal compensation flexure assembly formed in a first structure that is a part of an optical element alignment assembly. The flexure assembly comprises a set of collinear flexures and a center stage coupled to the set of collinear flexures. The set of collinear flexures and the center stage are configured to limit distortions in one direction due to a temperature change in the first structure from affecting an optical element supported by the first structure.
In one embodiment, three or more such assemblies may completely support a second structure, e.g., an optical element or assembly, with respect to the first structure such that there are minimal internal stresses, and hence distortions, in the second structure in the presence of bulk temperature changes or substantial temperature differences between the structures.
Another aspect of the invention relates to a micromachined strain isolation flexure assembly formed in a first structure that is a part of an optical element alignment assembly. The flexure assembly comprises a set of collinear flexures and a center stage coupled to the set of collinear flexures. The set of collinear flexures and the center stage are configured to limit strains in one direction in the first structure from transferring to a second structure.
Three or more such assemblies may completely isolate a second structure, e.g., an optical element or assembly, with respect to the first structure such that there are minimal internal stresses, and hence distortions, in the second structure in the presence of mechanically or inertially induced distortions in the first structure.
Another aspect of the invention relates to a method of making a micromachined flexure assembly in a structure that is a part of an optical element alignment assembly. The method comprises using lithography to form a pattern on a substrate for the structure. The pattern outlines a set of collinear flexures and a center stage coupled to the set of collinear flexures. The method further comprises etching the substrate to form the structure according to the pattern.